Cenozoic folding and faulting in the south Aquitaine Basin (France)

19 Pages·2000·2.92 MB·English

JournalofStructuralGeology22(2000)627–645
www.elsevier.nl/locate/jstrugeo
Cenozoic folding and faulting in the south Aquitaine Basin
(France): insights from combined structural and paleostress
analyses
Muriel Rochera, Olivier Lacombea,*, Jacques Angeliera, Benoıˆt De(cid:128)ontainesa,
Franc(cid:176)ois Verdierb
aUniversite´P.&M.Curie,ESA7072,CNRS,LaboratoiredeTectoniqueQuantitative,Tr25-26,E1,Boıˆte129,4placeJussieu,75252ParisCedex
05,France
bGazdeFrance,DirectiondelaRecherche,De´partementRe´servoirsSouterrains,LaPlaineSaintDenis,France
Received22April1998;accepted23November1999
Abstract
New fieldwork, surface data (e.g. drainage network anomalies) and SPOT satellite imagery are combined with sub-surface
data (seismic profiles and drill-cores) to analyse the structural setting of the south Aquitaine Basin. Cenozoic paleostresses are
determined through inversion of fault slip and calcite twin data (quarries and drill cores), allowing reconstruction of the
Cenozoic structural and tectonic evolution. The main tectonic event, the NNE ‘Pyrenean compression’, from the Late
Cretaceous to the Oligocene, is responsible for thrusting and folding along N1108 axes and strike-slip reactivation of major
NNW and NE–SW faults. Some fold axes turn along NNW major wrench faults, and compression locally undergoes deviation
to ENE trends. NNE extension locally occurred at anticline hinges. After a minor WNW extension, a Miocene NNW
compression occurredandchangedintoaperpendicularENEextension,responsiblefornearlyN–Snormalfaulting.
These multiple states of stress reflect two major compressional events (NNE and NNW); their variety mainly reveals local
accommodation due to numerous inherited structures, in the general context of Eurasia–Africa convergence. 7 2000 Elsevier
ScienceLtd. Allrightsreserved.
1. Structural setting and scope of the study northward migration of the foreland Pyrenean flexure
accompanied the setting of the Pyrenean thrust units,
The structure of the South-Aquitaine Basin (north- successively incorporating and deforming foredeep
western Pyrenean foreland, France) is dominated by sediments (De´ramond et al., 1993).
N110-trending folds and thrusts (Fig. 1a), which The structure of the Aquitaine Basin is mainly inher-
formed during the so-called ‘Pyrenean tectonic phase’, ited from the Late Carboniferous Variscan orogeny,
consequent to the N–S convergence of Africa and Eur-
and from Mesozoic extensions, including Triassic basin
asia plates. Most anticlines correspond to north-ver-
formation, Jurassic extension and Middle Cretaceous
gent fault-propagation folds, striking parallel to the
dislocation of the Aquitaine Platform.
Pyrenean Belt. Following Mesozoic extensions (see
During the Late Paleozoic, the Variscan orogeny
below), this Cenozoic tectogenesis was dominated by
generated the Variscan Belt in Central France with
nearly N–S compression, which prevailed from the
folding trending N110. Accordingly, N050–070 and
Late Cretaceous to the Oligocene. The progressive
N030 sinistral and N150 dextral faults developed in
the Aquitaine basement (Debelmas, 1986; Lefort,
1989).
*Correspondingauthor.
E-mailaddress:[email protected](O.Lacombe). The NNE-trending extension (read: NNE–SSW) re-
0191-8141/00/$-seefrontmatter72000ElsevierScienceLtd.Allrightsreserved.
PII: S0191-8141(99)00181-9
628 M.Rocheretal./JournalofStructuralGeology22(2000)627–645
sponsible for the formation of the Aquitaine Basin de´collement of the cover occurred in the western part
appeared as early as the Permian (Winnock, 1974; of the Aquitaine Basin where Triassic deposits were
Boillot, 1984; Autran and Cogne´, 1980). The Arzacq thick (that is above the Triassic paleo-basins).
and northern sub-basins (forming the Aquitaine Basin) During the Late Jurassic, the extension was trending
began to develop in response to a high rate of ten- nearly NE–SW, with WNW normal faults and NE–
sional subsidence, each one being split into several sec- SW transfer faults. The first migration of Triassic salts
ond-order basins by N160 sinistral faults. The piano- occurred along the WNW ridges, as, for example, the
key type di(cid:128)erential tilting along such fault lines deli- Audignon anticline (Fig. 1) (Ste´vaux and Zolnaı¨,
mit future major paleogeographic domains. In the 1975).
Arzacq basin, Triassic formations (sandstones, evapor- From Barremian to Albian, a NW-trending exten-
ites and clays) become thinner toward the north and sion was associated with the paroxysmal dislocation of
the east (Winnock, 1974). This Late Paleozoic–Early the Aquitaine Basin (Brunet, 1991). The Iberia–Eura-
Mesozoic structural pattern had important conse- sia boundary underwent a left-lateral movement as-
quences for the forthcoming Pyrenean deformation: sociated with the opening of the North Atlantic
Fig.1.(a)Generalstructuralsketchmapofthestudiedarea.Tectonicsitesaresituatedoneacharrowheadandnumbered: (1)Cros,(2)Tercis,
(3):Arcet,(4)and(5)respectively,wellsAandBofSiougos,(6)LaMe´nie`re,(7)Sainte-Suzanne,(8)Loubieng,(9)Bougue,(10)Sarraziet,(11)
Vic-Bilh,(12)Vive`s.(b)Geologicalcross-sectiondeducedfromECORS-Arzacqline(ECORSPyreneesteam,1988)elongatedtotheLandes.
M.Rocheretal./JournalofStructuralGeology22(2000)627–645 629
Ocean, the Bay of Biscay, and the Parentis basin be done. It is known that the minor fault patterns
(Mid-Aptian). This left-lateral movement resulted in usually reflect the regional tectonic style, so we use this
the development of pull-apart basins associated with opportunity to reconstruct tectonic mechanisms. Pro-
N140-trending sinistral faults and N060-trending nor- vided that local stress perturbations can be identified,
mal faults (Brunet, 1991). Then, from Albian to Late the analysis of minor fracture patterns thus constitutes
Cretaceous, the N110 faults underwent a sinistral a powerful tool to reconstruct the far-field paleo-
movement. stresses (see Section 3).
Finally, during the uppermost Cretaceous and Ceno- Such a multisource analysis brings strong constraints
zoic, in the Aquitaine basin, Mesozoic NNW- and on both the regional structural evolution through time
NE–SW-trending faults were reactivated as strike-slip and the kinematics of the major faults, even though
faults along which evaporites migrated, and fold– they are presently hidden by overlying sediments. As
thrust systems developed parallel to the N100–N110 an example, a single regional compressional strike-slip
Variscan Belt. state of stress accounts for the development of most
In this paper, we focus on the development of the folds, reverse and strike-slip faults and tension gashes,
northernmost structures of the Pyrenean belt. Recent although these features are in fact observable in very
studies showed that the present-day stress remains few outcrops. As a result, paleostress reconstructions
dominated by nearly N–S compression. However, the lead to more regionally significant information than
stress field is either perturbed (e.g. s (cid:255)s permu- that provided by the sole local observation of defor-
1 2
tations) or has deviated close to major inherited struc- mation.
tural features (Bell et al., 1992). By means of
paleostress reconstructions, we aim to evaluate how 2.1. Drainage pattern anomalies
long the Pyrenean compression prevailed, and how
much it was influenced by possible kinematic changes. In the flat Aquitaine region where scarps are scarce,
This implies a focus on the nature, extent and origin the analysis of the hydrographic network is a useful
of paleostress perturbations. The scope of this study is tool for structural mapping. It is known that studies of
to provide new insights on the structural evolution of drainage patterns bring significant contribution to
the Aquitaine Basin since the beginning of the Pyre- structural analyses (e.g. De(cid:128)ontaines and Chorowicz,
nean collision, with emphasis on the relationships 1991), even where structures are buried (Scanvic,
between major structures and paleostress orientations. 1983). The development of the drainage network is
To fulfil these requirements, a thorough reconstruc- influenced by external factors (e.g. climate, forest, agri-
tion of tectonic paleostress patterns was indispensable. cultural practice), internal factors (e.g. lithology, dip-
In the region of Mont-de-Marsan, because pre-Mio- slope, diapirs, folds and faults), and composite factors
cene natural scarps are few, we analysed tectonic fea- (e.g. eustatic changes).
tures, such as striated faults and calcite twins, in both In Fig. 2(a), we transferred the drainage network
quarries and drill-cores. For a better understanding of extracted from the I.G.N. 1/25000 topographic maps
the relationships between paleostresses, wrench faults of the Landes region, supplemented with temporary
and fold–thrust systems, we additionally investigated water courses and backwater deduced from topogra-
an exposed anticline genetically similar to the com- phy, from which we extracted rectilinear and curvi-
monly buried structures—the Sainte-Suzanne anti- linear drainage pattern anomalies. Anticlines and
cline—located along the North Pyrenean Front synclines generate a particular network (De(cid:128)ontaines,
(Fig. 1). 1990), including annular drainage and curvilinear
anomalies on their termination; faulting and lithology
generate rectilinear drainage anomalies.
2. Structural analysis from sub-surface and surface data Drainage anomalies occur along various directions:
N–S, N040, E–W, N120 and N160 (Fig. 2b). Because
Because most tectonic features are buried beneath bedding planes are frequently trending nearly N110,
post-collisional Miocene to Quaternary deposits, our parallel to the anticline axes, it is di(cid:129)cult to discrimi-
structural analysis combined fieldwork, surface investi- nate the E–W to N120 waterways following beds from
gations from analyses of drainage pattern anomalies those following faults.
and SPOT satellite imagery, with subsurface data, Three regions called A, B and C, separated by eva-
including seismic reflection profiles and information poritic ridges called ‘Celtaquitaine flexure’ CF (Labrit,
from drill-cores. This allowed us to improve the re- Roquefort, Cre´on, Barbotan anticlines) and ‘Gasconne
gional structural geology map. However, the continu- flexure’ GF (Tercis, Montfort, Audignon anticlines),
ity between major fault segments was di(cid:129)cult to are shown on Fig. 2. In the A region, major structures
establish from sparse field observations, so that a trend E–W to N120 (strata limits, folds and thrusts);
thorough mapping of the deformation field could not few rectilinear anomalies trend N–S. The B region dis-
630 M.Rocheretal./JournalofStructuralGeology22(2000)627–645
plays N040 rectilinear anomalies (like that near Tar- We used two scenes approximately in the same area
tas), some N–S anomalies (e.g. guiding of the Adour (KJ: 38-261). The region covered by each image is
River, arrow Fig. 2) and few N140 and E–W 60(cid:2)60km in size (Fig. 1a).
anomalies. The C region is bounded on the NE and SPOT analysis in a temperate area is limited by
the SW by NNW fault zones. Some N–S, N040 and dense groundcover in the NW, and intense agricultural
E–W anomalies are also identified. Numerous curvi- activity, complicating the interpretation. However,
linear anomalies exist near the Bastennes diapir struc- these images allowed an accurate location of folds and
ture (BD). major faults, especially taking into account flanks, pla-
A limit of this surface investigation is imposed by teau and crest morphologies (Fig. 3). Fault types have
the local scarceness of the drainage network, as in A been identified from visible o(cid:128)sets (folds, other faults,
and B areas (Fig. 2a). It is thus di(cid:129)cult to recognize bedding) and from the dip of the fault planes. These
the tectonic structures in these zones. Satellite imagery images thus provided limited, but useful information
provides additional information on subsurface struc- on both the geometry and the kinematics of regional
tures. structures.
As for drainage network studies, we divided the
2.2. SPOT satellite images region into A, B and C areas. The crescent-shaped
Audignon anticline separates B and C regions (Fig. 3).
Panchromatic SPOT images are in the visible band In the B region (Armagnac basin), we observe
of wavelength, from 0.51 to 0.73mm. Their major ad- N110-trending folds associated with parallel thrusts.
vantage is the accuracy, with 10-m ground resolution. Few NNW dextral faults and N–S faults occur. Some
Fig.2.(a)DrainagenetworkextractedfromIGN1:25000mapsandcompletedwithtemporarywatercoursesandbackwaterdeducedfromtopo-
graphy;(b)rectilinearandcurvilinearanomaliesofdrainagenetwork.A,B,C:regionsdefinedinthetext;GF:‘Gasconneflexure’;CF:‘Celtaqui-
taineflexure’;BD:Bastennesdiapir.Arrow:seetext.Structuralfeatures:samekeyasFig.1(a).
M.Rocheretal./JournalofStructuralGeology22(2000)627–645 631
N040 fractures, one of them guiding the Midouze strike-slip faults, trending approximately N170 and
watercourse, are identified. The C region (Arzacq N050, N030 and E–W, which resemble two systems of
Basin) is separated from the southern Mauleon Basin conjugate faults related to N015 and N060 com-
(D) by the N100 North Pyrenean Frontal thrust, cut pressions. The N015 s direction was suspected
1
by N160 and N040 strike-slip faults. The C region is throughout the area covered by the image; we infer
characterized on the west by folds with N110 axes and that it reflects the major regional NNE Pyrenean com-
parallel faults, generally corresponding to thrusts, and pression. The nearly N060 s direction was recognized
1
the NNW Boos fault zone. The eastern part of this near some NNW major faults. This compression prob-
region displays NNW major faults (for example, the ably results from a local deviation of the NNE com-
Larcis fault). The bending of fold axes near these pression near the NNW-trending fault zones. This
faults and the o(cid:128)sets of fold axes suggest a dextral interpretation, checked with paleostress reconstruc-
movement. tions, favours local stress perturbations during a single
Sets of subvertical faults (with rectilinear trace) with major tectonic event, rather than distinct events.
consistent azimuths and dihedral angles of about 608 Drainage network and SPOT imagery analyses thus
are interpreted as conjugate strike-slip faults; they are provided consistent and complementary structural in-
generally located on the anticlines, and probably formation. Fieldwork and seismic studies allowed us to
formed during and after folding. The mean direction identify and distinguish superficial and buried struc-
of compression (s ) is thus easily identified (Fig. 3b), tures.
1
even though field analysis is required for confirmation
(see below). To the north, two fracture sets, trending 2.3. Seismic profiles
N170 and N050, form a couple of conjugate strike-slip
faults consistent with a N015 compression. To the The N–S cross-section of the Aquitaine region
south and the southeast there are four sets of supposed (Fig. 1b), as revealed by deep seismic profiling
Fig.3.(a)OneSPOTimageofthearea(seelocationonFig.1a);(b)structuresdeducedfromSPOTimage,ands trendsdeducedfromstrike-
1
slipfaults.Structuralfeatures:seeFig.1(a),andA,B,C,GF,CF:seeFig.2.NPF:NorthPyreneanFront;D:seetext.
632 M.Rocheretal./JournalofStructuralGeology22(2000)627–645
(ECORS Pyrenees team, 1988), show that the Pyrenees 3. Mechanical analysis of brittle tectonics
are a doubly-vergent belt. Northward from the Axial
Zone, the section cuts the Mauleon Basin, the North 3.1. Inversion of fault slip data
Pyrenean fault zone and the North Pyrenean thrust
zone (from St-Palais to Ste-Suzanne), then the Arzacq In fault tectonics, the inverse problem consists of
basin and the Landes shoal (our regions of interest). reconstructing paleostresses based on measurements of
From the North Pyrenean thrust to the Landes, the the directions and sense of slip along a large number
fold structure is rather regular, with an average wave- of minor faults (e.g. Carey and Brunier, 1974; Etcheco-
length of 20km. Most anticlines correspond to fault- par et al., 1981; Angelier, 1984, 1990). The basic prin-
propagation folds associated with north-vergent thrusts ciple is that the direction and sense of the observed
and a few backthrusts; some have localized diapirism. motion along a fault plane (i.e. slickenside lineations,
Thrusts may reach the surface or remain overlain by Fig. 6a) are parallel to the maximum shear stress
foreland Neogene deposits. They initiated upon base- exerted on it.
ment faults and root in the Triassic evaporitic for- The computerized inversion of fault slip data yields
mations, which act as a de´collement (Mathieu, 1986), the reduced stress tensor, including the orientation of
or correspond to inverted N100 and N160 basement the principal stresses s , s and s (cid:133)s rs rs , com-
1 2 3 1 2 3
faults (Fig. 1b). pression positive) and the F ratio
Seismic reflection profiles (Fig. 4) show that the (cid:133)F(cid:136)(cid:133)s (cid:255)s (cid:134)=(cid:133)s (cid:255)s (cid:134), 0RFR1). In this study we
2 3 1 3
Siougos anticline includes both a thrust and back- used the INVD method of Angelier (1984, 1990). In
thrusts (Fig. 4a–c). As deduced from pinching out, the case of polyphase faulting, the mechanically con-
unconformities and variations in sediment thickness, sistent fault slips are gathered, taking into account
folding was syn-depositional from the Late Cretaceous chronological observations. Such observations deal
to the Oligocene (see below). Normal faulting occurs withsuperimposedstriationsonfaultsurfaces(Fig.6a),
near the top of this anticline. The E–W seismic profiles crosscutting relationships between faults, evidence for
(parallel to fold axis) cut numerous flower structures syn-depositional tectonics, time relationships between
with reverse or normal o(cid:128)sets (Fig. 4d). Around these faulting and folding (Fig. 6b), etc.
wrench zones, bulging of the Triassic formations and The tensors are calculated for each subset of fault
the overlying strata suggests a probable push-up of slips. Where tilted bedding is observed (as a result of
Triassic evaporites and clays. folding), several cases deserve consideration because
This structural study was completed with structure faults may have formed before, during or after folding.
contour maps of the K/T limit, based on seismic pro- Following Anderson (1951), we assumed that one of
files (GDF, unpublished data); this study provides in- the three principal stress axes of a tensor is generally
formation about location of folds and occurrence of vertical. If a fault set formed before folding and was
major faults, consistent with surface data. secondarily tilted with the bedding, the tensor calcu-
As a result, combined fieldwork, seismic and surface lated on this set does not display a vertical axis.
data studies allowed us to construct an improved Instead, one of the stress axes is generally found per-
structural geology map (Fig. 5), which contains much pendicular to bedding, whereas the two others lie
more detail than Fig. 1(a). within the bedding plane. Accordingly, the conjugate
Fig.4.Cross-sectionsofSiougosstructurededucedfromseismicreflectionprofiles.(a–c)N–Ssections;(d)E–Wsection.
M.Rocheretal./JournalofStructuralGeology22(2000)627–645 633
fault systems do not display vertical planes of sym- twinning t ; if not, the e-plane remains untwinned.
a
metry. In such a case, it is necessary to backtilt the This yield stress value is nearly independent of tem-
whole system (faults, tensor and bedding) in order to perature, confining and fluid pressure, but depends on
put it back into its initial position. Fig. 6(b) shows an grain size (Rowe and Rutter, 1990). Following Tullis
example of backtilting of a fault set measured in the (1980), Laurent (1984), and Craddock et al. (1993), we
Tercis quarry. Note, however, that to be correct such have adopted a value of 10MPa for t for a mean
a
a backtilting requires that folds be cylindrical in type grain size of about 200–300mm. For each sample, cal-
with horizontal axes. Under these restrictions, this cite twin data were collected using a Universal Stage
kind of analysis provides valuable information on the within three mutually perpendicular thin sections.
relative chronology between faulting events. In this paper, determination of paleostresses from
twin data was performed through numerical inversion
3.2. Inversion of calcite twin data (Etchecopar, 1984), which has already led to consistent
paleostress reconstructions in nonmetamorphic, weakly
At low pressure and temperature, calcite aggregates deformed carbonate cover rocks (e.g. Lacombe et al.,
deform primarily by twinning. Twin lamellae (Fig. 6c) 1992; Rocher et al., 1996). The tensor solution must
result from the simple shear of part of host crystals theoretically meet the major requirement that all the
along f011(cid:22)2g crystallographic planes, called e-twin twinned planes consistent with it should sustain a t
s
planes (e.g. Turner et al., 1954). Twinning occurs if the value larger than that exerted on all the untwinned
resolved shear stress t acting along each e-plane is lar- planes. Twin data inversion provides the orientation of
s
ger than (or at least equals) the yield stress value for the principal stresses responsible for twinning (Turner,
Fig.5.Syntheticstructuralmapofthestudiedregion(comparetoFig.1a).Structuralfeatures:samekeyasFig.1(a).
634 M.Rocheretal./JournalofStructuralGeology22(2000)627–645
Fig.6.Photographs.A—Cross-cuttingstriaeonalargefaultplanewithinUrgonianlimestonesonSainte-Suzanneanticline.B—Tiltedstrike-slip
faultsinTercisquarry,andprincipleofbacktiltingforfaultsetmeasured(S :beddingplane).C—Exampleofmeasuredmechanicaltwinsetsin
0
calcite. D—Reverse fault in the La Me´nie`re quarry; E—Flower structure in La Me´nie`re quarry; F—Stylolites associated with bedding-parallel
slipsinLoubiengquarry;G—LatenormalfaultsinLoubiengquarry.Diagrams:thincurvesrepresentfaultplanes,anddotswithdoublearrows
(left- or right-lateral) or simple ones (centrifugal-normal; centripetal-reverse) indicate slickenside lineations. Stars indicate stress axes with five
points(s),fourpoints(s),andthreepoints(s).Emptysquaresrepresentpolestotensiongashes.Fulllozengesrepresentstyloliticpeaks.Bed-
1 2 3
dingplanes(S)shownasdashedlines.
0
M.Rocheretal./JournalofStructuralGeology22(2000)627–645 635
1953; Dietrich and Song, 1984; Pfi(cid:128)ner and Burkhard, As shown by Rocher (1999), determination of super-
1987; Lacombe and Laurent, 1992; Craddock et al., imposed stress tensors by the Etchecopar method
1993), the F ratio, and an intra-program value of the (1984) is particularly reliable (less than 58 of error on
yield stress, t , (Etchecopar, 1984; Tourneret and the principal stress axes orientations, and less than 0.1
a0
Laurent, 1990), which corresponds to the lowest t on F). The (cid:133)s (cid:255)s (cid:134) magnitude is generally well
s 1 3
value sustained by the twinned planes accounted for defined for the first tensor determined, but separation
by the tensor solution, for a di(cid:128)erential stress (cid:133)s (cid:255)s (cid:134) of states of stress may induce a maximum error of
1 3
scaled to 1 (cid:137)(cid:133)s (cid:255)s (cid:134)(cid:3) (cid:136)1]. The fifth parameter 50% in the magnitude of the successive tensors; these
1 3
(cid:133)s (cid:255)s (cid:134) (MPa) of the tensor can thus be determined values are therefore considered herein as rough esti-
1 3
as follows: mates of di(cid:128)erential stress magnitudes.
In case of folding, the principle of backtilting stress
t 10
(cid:133)s (cid:255)s (cid:134)(cid:136) a (cid:2)(cid:133)s (cid:255)s (cid:134)(cid:3) (cid:136) : (cid:133)1(cid:134) tensors is the same as for fault slip analysis: the back-
1 3 ta0 1 3 ta0 tilted solution is retained provided that the orientation
of stress axes is significantly improved with respect to
If many twins are found that are not consistent with a
the horizontal and/or vertical.
stress tensor, the process is repeated with these incon-
sistent twinned planes and the whole set of untwinned
planes. In case of polyphase tectonism, such a separ- 3.3. Paleostress reconstructions in the Mont-de-Marsan
ation of superimposed stress tensors has provided con- region
sistent results with independent stress reconstructions
from polyphase fault sets in the same sites (e.g. In the Mont-de-Marsan region, paleostresses were
Lacombe et al., 1992). reconstructed using both fault slips and calcite twins.
Fig.7.Compressionalstatesofstressdeducedfromcalcitetwinsandfaultslips.Largediagrams:faults,smalldiagrams:twins(samekeyasFig.
6).
636 M.Rocheretal./JournalofStructuralGeology22(2000)627–645
Four important sites located on N110-trending anti- diapir. Most faults are strike-slip or reverse in type
clines allowed observation of fracture patterns: the and associated with layer-parallel slips, consistent with
quarries of Cros, Tercis and Arcet, and the cores of a NNE compression predating folding: we presently
two wells on the Siougos anticline (Figs. 7 and 8, observe N120 normal faults dipping 708 to the south,
Table 1). Ten calcite samples were collected in the and N020 normal faults dipping 458 to the east or to
Arcet, Sarraziet, Vive`s and Bougue quarries, and in the west, which respectively correspond after backtilt-
the cores from Siougos (Figs. 7 and 8, Table 2). ing to N110 reverse faults, and N030 dextral and N170
Fault measurements in the Cros quarry (Fig. 1a, sinistral strike-slip faults (Fig. 6b). Some strike-slip
Table 1) were made on Coniacian limestones, dipping and reverse faults yield a NNE compression post-dat-
308 to the north. Conjugate wrench faults and reverse ing folding (these faults formed in the position in
faults associated with stylolites indicate a NNE and a which they are presently observed). Many stylolitic
NNW compression (Fig. 7). Crosscutting striations on joints, with dips from 0 to 908, are consistent with the
fault planes indicate that the NNW compression post- NNE compression; oblique ones having formed during
dated the NNE compression. The NNW compression strata tilting. This indicates that the NNE com-
(with reverse faults, Fig. 7) progressively evolved into pression, which is perpendicular to fold axis, prevailed
a strike-slip regime with s trending NNW and s before, during and after strata tilting and is therefore
1 3
trending ENE (cid:133)s (cid:255)s permutation) and finally into responsible for both folding and contemporaneous
2 3
an ENE extension (cid:133)s (cid:255)s permutation, Fig. 8), as faulting. Some other post-folding states of stress were
1 2
deduced from evolution of pitch of striations on a reconstituted (Table 1): a NNW compression, followed
same fault plane, or along parallel fault planes (Gui- by a perpendicular ENE extension, then a minor ENE
mera`, 1984). Minor newly formed normal faults related compression. A NW–SE extension was also reconsti-
to this late extension cut all previous structures. tuted but relative dating of this event was not possible.
In the Tercis quarry (Fig. 1a, Table 1), Campanian The Arcet quarry (Fig. 1a, Table 1) consists of
to Maastrichtian vertical limestone strata trend N100. Danian dolomitised limestones with bedding striking
Some Triassic clays were observed on the outcropping N090 with dip of 308N, unconformably covered by
core of the Tercis fold, relayed to the east by the Dax Oligocene deposits. Large vertical faults correspond to
Fig.8.Extensionalstatesofstressdeducedfromcalcitetwinsandfaultslips.(SamekeysasFigs.6and7.)